Origin and fate of a number of substances that can be identified in the enamel organ with the electron microscope are not evident at all from ordinary morphologic studies. The use of radiolabeled tracers is helpful in elucidating their origin, but of little use when studying their subsequent fate, partly because of a lack of sufficient specificity and resolving power of the radioautographic technique. The use of tracer proteins such as horse radish peroxidase promises to be superior to radioautographic techniques in this respect when used with the appropriate controls. Rats are injected with the tracer protein, killed at appropriate time intervals, and location of the protein is revealed by an enzyme reaction. Tracer distribution is studied and is interpreted in terms of movement of large molecular weight substances through extracellular spaces, compartmentalization of the extracellular space, and interactions between cells and extracellular substances.